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Regularized focusing inversion of time-lapse electrical resistivity data: an approach to parametrize the minimum gradient support functionalhttp://hdl.handle.net/2268/178763
Title: Regularized focusing inversion of time-lapse electrical resistivity data: an approach to parametrize the minimum gradient support functional
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<br/>Author, co-author: Nguyen, Frédéric; Hermans, Thomas
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<br/>Abstract: Inversion of time-lapse resistivity data allows obtaining ‘snapshots’ of changes occurring in monitored systems for applications such as aquifer storage, geothermal heat exchange, site remediation or tracer tests. Based on these snapshots, one can infer qualitative information on the location and morphology of changes occurring in the subsurface but also quantitative estimates on the degree of changes in certain property such as temperature or total dissolved solid content. Analysis of these changes can provide direct insight into flow and transport and associated processes and controlling parameters. However, the reliability of the analysis is dependent on survey geometry, measurement schemes, data error, and regularization. Survey design parameters may be optimized prior to the monitoring survey. Regularization, on the other hand, may be chosen depending on available information collected during the monitoring. Common approaches consider smoothing model changes both in space and time but it is often needed to obtain a sharp temporal anomaly, for example in fractured aquifers.
We here propose to use the alternative regularization approach based on minimum gradient support (MGS) (Zhdanov, 2002) for time-lapse surveys which will focus the changes in tomograms snapshots. MGS will limit the occurrences of changes in electrical resistivity but will also restrict the variations of these changes inside the different zones. A common difficulty encountered by practitioners in this type of regularization is the choice of an additional parameter, the so-called , required to define the MGS functional. To the best of our knowledge, there is no commonly accepted or standard methodology to optimize the MGS parameter . The inversion algorithm used in this study is CRTomo (Kemna 2000). It uses a Gauss-Newton scheme to iteratively minimize an objective function which consists of a data misfit functional and a model constraint functional. A univariate line search is performed at each Gauss-Newton iteration step to find the optimum value of the regularization parameter  which minimizes the data misfit as a function of  while the data misfit is above the desired value and yields the desired target misfit (root-mean square value of error-weighted data misfit equal to 1) at the last iteration for a maximum value of . We propose here to optimize the  of the MGS functional by considering a univariate line search at the first iteration to find the  that minimizes the data misfit. The parameter is then kept constant during the Gauss-Newton iterative scheme. In this contribution, we validate our approach on a numerical benchmark and apply it successfully on a case study in the context of salt tracers in fractured aquifers.
Zhdanov M.S. 2002. Geophysical Inverse Theory and Regularization Problems. Elsevier, Amsterdam, 628 p.
Kemna A. 2000. Tomographic Inversion of Complex Resistivity - Theory and Application. PhD Thesis, Ruhr University Bochum.Variogram-based inversion of time-lapse electrical resistivity data: development and application to a thermal tracing experimenthttp://hdl.handle.net/2268/178762
Title: Variogram-based inversion of time-lapse electrical resistivity data: development and application to a thermal tracing experiment
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<br/>Author, co-author: Hermans, Thomas; Nguyen, Frédéric
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<br/>Abstract: Electrical resistivity tomography (ERT) has become a popular imaging methodology in a broad range of applications given its large sensitivity to subsurface parameters and its relative simplicity to implement. More particularly, time-lapse ERT is now increasingly used for monitoring purposes in many contexts such as water content, permafrost, landslide, seawater intrusion, solute transport or heat transport experiments.
Specific inversion schemes have been developed for time-lapse data sets. However, in contrast with static inversions for which many techniques including geostatistical, minimum support or structural inversion are commonly applied, most of the methodologies for time-lapse inversion still rely on non-physically based spatial and/or temporal smoothing of the parameters or parameter changes.
In this work, we propose a time-lapse ERT inversion scheme based on the difference inversion scheme. We replace the standard smoothness-constraint regularization operator by the parameter change covariance matrix. This operator takes into account the correlation between changes in resistivity at different locations through a variogram computed using independent data (e.g., electromagnetic logs). It may vary for subsequent time-steps if the correlation length is time-dependent.
The methodology is first validated and compared to the standard smoothness-constraint inversion using a synthetic benchmark simulating the injection of a conductive tracer into a homogeneous aquifer inducing changes in resistivity values of known correlation length. We analyze the influence of the assumed correlation length on inversion results. Globally, the method yields better results than the traditional smoothness constraint inversion. Even if a wrong correlation length is assumed, the method performs as well as the smoothness constraint since the regularization operator balances the weight given to the model constraint functional in the objective function.
Then the methodology is successfully applied to a heat injection and pumping experiment in an alluvial aquifer. The comparison with direct measurements in boreholes (temperature loggers and distributed temperature sensing optic fibres) shows that ERT-derived temperatures and breakthrough curves image reliably the heat plume through time (increasing part of the curve, maximum and tail are correctly retrieved) and space (lateral variations of temperature are observed) with less spatial smoothing than standard methods.
The development of new regularization operators for time-lapse inversion of ERT data is necessary given the broad range of applications where ERT monitoring is used. In many studies, independent data are available to derive geostatistical parameters that can be subsequently used to regularize geophysical inversions. In the future, the integration of spatio-temporal variograms into existing 4D inversion schemes should further improve ERT time-lapse imaging.VADOSE ZONE STUDIES AT AN INDUSTRIAL CONTAMINATED SITE: THE VADOSE ZONE MONITORING SYSTEM AND CROSS-HOLE GEOPHYSICShttp://hdl.handle.net/2268/178428
Title: VADOSE ZONE STUDIES AT AN INDUSTRIAL CONTAMINATED SITE: THE VADOSE ZONE MONITORING SYSTEM AND CROSS-HOLE GEOPHYSICS
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<br/>Author, co-author: Fernandez de Vera, Natalia; Beaujean, Jean; Jamin, Pierre; Nguyen, Frédéric; Dahan, Ofer; Vanclooster, Marnik; Brouyère, Serge
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<br/>Abstract: In situ vadose zone characterization is essential to improve risk characterization and remediation measures for soil and groundwater contamination. However, most available technologies have been developed in the context of agricultural soils. Most of these methodologies are not applicable at industrial sites, where soils and contamination
differ in origin and composition. In addition, they are applicable only in the first meters of soils, leaving deeper vadose zones with lack of information, in particular on field scale heterogeneity.
To overcome such difficulties, a vadose zone experiment has been setup at a former industrial site in Belgium.
Industrial activities carried out on site left a legacy of soil and groundwater contamination in BTEX, PAH, cyanide and heavy metals. The experiment involves the combination of two techniques: the Vadose Zone Monitoring System (VMS) and cross-hole geophysics.
The VMS allows continuous measurements of water content at different depths of the vadose zone (Dahan et al., 2009). In addition, it provides the possibility of pore water sampling at different depths. The system is formed by a flexible sleeve installed in a slanted borehole (Fig. 1) and containing monitoring units along its depth (Fig. 2). The flexible sleeve contains three types of monitoring units in the vadose zone: Time Domain Transmissometry (TDT), which allows water content measurements; Vadose Sampling Ports (VSP), used for collecting water samples coming from the matrix; and the Fracture Samplers (FS), which are used for retrieving water samples from the fractures. Cross-hole electrical tomography measurements are carried providing detailed spatial patterns about electrical properties of the subsurface. Such properties are related with subsurface heterogeneities, water content and solute concentrations.
Two VMS were installed on site, together with four vertical boreholes containing electrodes for geophysical measurements. The site has been monitored under natural recharge conditions during the summer, autumn and winter. Results show reactions in the soil at depths up to 6m as a consequence of rainfall infiltration and groundwater level fluctuations. In addition, the chemistry of the soil water changes with depth and water infiltration. Background images obtained from geophysical measurements show a highly conductive subsurface due to the lithologies and the high mineralization of the water in the vadose zone. The combination of cross-hole geophysics with the VMS has provided an effective tool for characterizing the chemistry and the structure of the vadose zone.Sablières de ROSSART à Flémalle - Piézomètres de contrôle - Prélèvements d’eau des 22 et 24 septembre 2014http://hdl.handle.net/2268/176660
Title: Sablières de ROSSART à Flémalle - Piézomètres de contrôle - Prélèvements d’eau des 22 et 24 septembre 2014
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<br/>Author, co-author: Orban, Philippe; Dassargues, AlainCarrière de CORREUX à Sprimont - Piézomètre de contrôle - Interprétation des analyses des échantillons d’eau souterraine prélevés le 23 septembre 2014http://hdl.handle.net/2268/176655
Title: Carrière de CORREUX à Sprimont - Piézomètre de contrôle - Interprétation des analyses des échantillons d’eau souterraine prélevés le 23 septembre 2014
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<br/>Author, co-author: Orban, Philippe; Dassargues, AlainA heat and dye tracer test for characterizing and modelling heat transfer in an alluvial aquiferhttp://hdl.handle.net/2268/176622
Title: A heat and dye tracer test for characterizing and modelling heat transfer in an alluvial aquifer
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<br/>Author, co-author: Klepikova, Maria; Wildemeersch, Samuel; Jamin, Pierre; Orban, Philippe; Hermans, Thomas; Brouyère, Serge; Dassargues, Alain
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<br/>Abstract: Using heat as an active tracer is a topic of increasing interest with regards to characterizing shallow aquifers for ATES (Aquifer Thermal Energy Storage) systems. In this study, we investigate the potential interest of coupling simultaneous heat and dye tracer injection tests for characterization of an alluvial aquifer. The study site is located near Liege in the alluvial aquifer of the Meuse River, Belgium. The tracing experiment consisted in simultaneously injecting heated water and a dye tracer in a piezometer and monitoring the evolution of groundwater temperature and tracer concentration in the recovery well and in nine monitoring wells located according to three transects with regards to the main groundwater flow direction.
The breakthrough curves measured in the recovery well showed that heat transfer in the alluvial aquifer is slower and more dispersive than solute transport. Recovery is very low for heat while in the same time it is measured as relatively high for the solute tracer. This shows how heat diffusion is larger than molecular diffusion, implying that exchange between groundwater and the porous medium matrix is far more significant for heat than for solute tracers.
In a first step, temperature and concentrations in the recovery well are used for estimating the specific heat capacity with an energy balance calculation and the estimated value is found to be consistent with those found in the literature. Then, the measured temperature breakthrough curves in the piezometers are used for constraining the heat transport model. They are highly contrasted with what would be expected in an ideal layered aquifer. They reveal strongly unequal lateral and vertical components of the transport mechanisms. A preliminary interpretation of these temperature breakthrough curves is provided with first results from the model. Then it will allow for estimating the entire set of heat transfer parameters and their spatial distribution by inverse modelling.
The developed concepts and tests may lead to real projects of various extents that can be now optimized by the use of a rigorous and efficient methodology at the field scale.SEM and EDS observations of carrollite bioleaching with a mixed culture of acidophilic bacteriahttp://hdl.handle.net/2268/176114
Title: SEM and EDS observations of carrollite bioleaching with a mixed culture of acidophilic bacteria
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<br/>Author, co-author: Nkulu, Guy; Gaydardzhiev, Stoyan; Mwema, Edouard; Compère, Philippe
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<br/>Abstract: Bioleaching of high purity carrollite minerals with mesophilic bacteria was carried out and monitored by observations in scanning electron microscopy (SEM) and elemental X-ray microanalysis (EDS) to provide evidence of the interaction pattern between carrollite and microorganisms. A bacterial consortium involving three different acidophilic chemolithotrophs was adopted. The evolution of the surface topography, inside alteration effects and elemental composition of the mineral with leaching time was followed. It could be postulated that bacterial adhesion takes place on the mineral surface, resulting in the formation of dissolution pits of various shapes and continues by boring elongated channels deep inside the mineral grains. Enhanced concentration of ferric iron and sulphur could be assumed in vicinity of the zones where mineralized polymer substances are precipitated. It could be inferred that carrollite dissolution is governed by cooperative bioleaching involving oxidation induced by bacteria attached to the surface and ferric iron re-oxidized by planktonic bacteria in suspension.Possibilities for Co(III) dissolution from an oxidized ore through simultaneous bioleaching of pyritehttp://hdl.handle.net/2268/176113
Title: Possibilities for Co(III) dissolution from an oxidized ore through simultaneous bioleaching of pyrite
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<br/>Author, co-author: Zeka, Leon; Lambert, Fanny; Frenay, Jean; Gaydardzhiev, Stoyan; Ndala, Augustin
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<br/>Abstract: Solubilisation of Co(III) from a heterogenite met in copper cobaltiferous oxide ore has been realized through reductive leaching using ferrous iron generated via bio-oxidation of pyrite. Biotic and abiotic experiments at various pulp densities and redox potentials have been performed and results compared. Cobalt leaching at elevated redox potential is possible, offering cost reduction benefits due to reduced consumption of ferrous iron. At elevated potential of 625 mV, however, the initial rate of cobalt leaching has been found as 115 mg/(g ore)⋅(24 h), lower than the rate of 865 mg/(g ore)⋅(24 h) registered at 505 mV. Less stochiometric amount of ferrous iron was required when cobalt leaching was coupled to pyrite bioleaching, with 75% of cobalt recovered for 12 h at the optimally found conditions. It could be inferred that the Fe3+–Fe2+ cycle exists and is efficiently maintained through bacterial presence in the studied system.Rapport d'activité n°1 du consortium Geolys - HGE-ULg dans le cadre du projet « Méthodologie de diagnostic environnemental autour de captages d’eau potabilisable sensibles qualitativement du point de vue du nitrate »http://hdl.handle.net/2268/175306
Title: Rapport d'activité n°1 du consortium Geolys - HGE-ULg dans le cadre du projet « Méthodologie de diagnostic environnemental autour de captages d’eau potabilisable sensibles qualitativement du point de vue du nitrate »
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<br/>Author, co-author: Huby, Mélanie; Orban, Philippe; Wittorski, Olivier; Piront, Laurent; Brouyère, SergeEtude géologique et géophysique en vue de la construction d'un bassin d'orage à Jupille, rue de Beynehttp://hdl.handle.net/2268/174396
Title: Etude géologique et géophysique en vue de la construction d'un bassin d'orage à Jupille, rue de Beyne
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<br/>Author, co-author: Delvoie, Simon; Collin, Frédéric
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<br/>Abstract: Une série de profils de résistivité électrique ont été réalisés en vue de repérer d'éventuels puits de mine au sein du bedrock houiller.Etude géologique et géophysique en terrain karstique du projet de lotissement à Heusy, avenue du Chênehttp://hdl.handle.net/2268/174394
Title: Etude géologique et géophysique en terrain karstique du projet de lotissement à Heusy, avenue du Chêne
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<br/>Author, co-author: Delvoie, Simon; Collin, Frédéric
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<br/>Abstract: Une série de profils de résistivité électrique ont été réalisés afin de préciser la délimitation des terrains carbonatés (Formation de Nèvremont) et de repérer d'éventuels édifices karstiques.Les nanoparticules et les micro-organismes au service de la dépollution des solshttp://hdl.handle.net/2268/174052
Title: Les nanoparticules et les micro-organismes au service de la dépollution des sols
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<br/>Author, co-author: Lavigne, Benoît; Lambert, StéphanieHydromechanical behaviour of interfaces in the framework of nuclear waste disposalhttp://hdl.handle.net/2268/173964
Title: Hydromechanical behaviour of interfaces in the framework of nuclear waste disposal
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<br/>Author, co-author: Dieudonné, Anne-Catherine; Salehnia, Fatemeh; Li, Xiang-Ling; Charlier, RobertAccounting for evolving pore size distribution in water retention models for compacted clayshttp://hdl.handle.net/2268/173775
Title: Accounting for evolving pore size distribution in water retention models for compacted clays
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<br/>Author, co-author: Della Vecchia, Gabriele; Dieudonné, Anne-Catherine; Jommi, Cristina; Charlier, RobertPhysicochemical observations during process water reuse in flotation of oxide copper-cobalt orehttp://hdl.handle.net/2268/173706
Title: Physicochemical observations during process water reuse in flotation of oxide copper-cobalt ore
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<br/>Author, co-author: Shengo, Michel; Gaydardzhiev, Stoyan; Kalenga, Pierre
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<br/>Abstract: Lab-scale flotation of copper and cobalt oxidized ore from the Luiswishi deposit in the Katanga province (DRC) has been done focusing on the influence of the degree of process water recycling on metals recovery. Results have shown that at 20 % recycling rate, the recovery of copper and cobalt in the cleaner stage have been respectively reduced to 23 % and 45 % in comparison to 53 % Cu and 63 % Co if process water has not been recycled. At 10 % recycling rate however, the system could still tolerate the effect from recycling, since Cu and Co recovery has remained respectively at acceptable levels of 52 % and 60 %. In order to explain the reasons for the observed worsening of flotation results if process water was recycled at rate higher than 20 %, electrochemical investigation of the pulp has been performed. It has been established that the chemical compounds formed in the pulp when process water was recycled, do influence the sulfidization of malachite and heterogenite and impact further collector adsorption. To confirm these assumptions, the effect from thiosulphate ions presence during the sulfidization of high purity malachite has been studied. It has been found, that thiosulphate ions depending on their concentration in water induce variations in pulp properties such as DO content, pH, Es and contribute to formation of undesirable chemical compounds on mineral surface as identified by DRIFT spectroscopy.Insights upon the adsorption mechanism of KAX on malachitehttp://hdl.handle.net/2268/173705
Title: Insights upon the adsorption mechanism of KAX on malachite
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<br/>Author, co-author: Kanda, Jean-Marie; Kongolo, Mukendi; Gaydardzhiev, Stoyan; De Donato, Philippe; Barres, Odile; Bastin, David
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<br/>Abstract: The adsorption patterns of potassium amylxanthate on the surface of high purity malachite subjected to prior sulfidization with NaSH have been followed and elucidated. DRIFTS spectroscopy has been used to trace the associated physicochemical alterations as well as the chemical species present on the mineral surface. Varying the collector concentration from 5.10-4 to 1.10-2 mol/L has led to different degree of surface coverage of the alkyl groups in combination with formation of organometallic hydrophobic complexes of CuX and CuX – X2 type. The results reveal that the CuX complex formed on the malachite surface without sulfidizing agent is more unstable and prone to dissolution-precipitation phenomena. Based upon the constructed adsorption isotherms and upon the degree of statistical coverage, different zones of mineral-reagent interactions could be distinguished. They correspond respectively to the monolayer, bi-layer and tridimensional condensation domains. In each of the identified zones mono or double layer adsorption patterns of the collector are proposed leading respectively to formation of surface compounds with different degree of hydrophobicity.Uncertainty in Training-Image Based Inversion of Hydraulic Head Data Constrained to ERT Data: Workflow and Case Studyhttp://hdl.handle.net/2268/173349
Title: Uncertainty in Training-Image Based Inversion of Hydraulic Head Data Constrained to ERT Data: Workflow and Case Study
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<br/>Author, co-author: Hermans, Thomas; Nguyen, Frédéric; Caers, Jef
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<br/>Abstract: In inverse problems, investigating the relationship between data and prior models and the uncertainty related to the posterior distribution of model parameters are as important as matching the data. In recent years, many efforts have been done to assess the posterior distribution of a given problem with reasonable computational costs through inversion techniques such as McMC. The derived posterior distribution is always dependent on the prior distribution. However, most of the studies ignore modeling the prior with realistic uncertainty. In this paper, we propose a workflow to assess the uncertainty of inversion of hydraulic heads data through the addition of electrical resistivity tomography (ERT) constraining data. The workflow is divided in three successive steps:
1) Construction of prior: we generate multiple alternative geological scenarios from literature data (architecture of facies) as well as site specific data (proportions of facies). Spatial uncertainty within each scenario is integrated hierarchically through geostatistics (multiple-point statistics simulation of facies constrained by ERT data as soft data).
2) Validation of prior scenarios: we transform prior facies scenarios into resistivity distribution scenarios through forward and inverse modeling. The scenarios are validated by comparison with field ERT data. The comparison is made through distance calculation and projection into a low dimensional space to calculate the probability of each scenario given field ERT data.
3) Matching dynamical data: we use the probability perturbation method, within each scenario, to integrate hydraulic heads to our models. We account for scenario probabilities, calculated in 2, in determining how many models per scenario we have to consider for building a reliable posterior distribution.
As an illustration, the method is applied on a field case study in an alluvial aquifer (Belgium) where we consider prior uncertainty related to the type of elements (gravel channels or bars) and to their size. This study shows the importance of considering the uncertainty of the prior in inverse problems as it has a strong influence on model predictions and decision-making problems.Contexte hydrogéologique de la commune de Sprimonthttp://hdl.handle.net/2268/173005
Title: Contexte hydrogéologique de la commune de Sprimont
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<br/>Author, co-author: Gesels, Julie; Gilson, Mylene; Brouyère, Serge; Ruthy, Ingrid
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<br/>Abstract: Cadre hydrogéologique de la commune de Sprimont : formations géologiques qui contiennent des ressources en eau intéressantes, variations de la profondeur d'eau dans les différentes nappes, volumes exploités et chimisme des eaux souterraines.
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<br/>Commentary: Cet article a été rédigé en 2012 à la demande de la commune de Sprimont et de la Région Wallonne (SPW). Il a été intégré dans un ouvrage destiné notamment au grand public et publié à l'occasion de l'édition 2014 de la Fête de la Pierre (Sprimont).Prospection géophysique de la zone faillée de Hockai dans la région de Malmedy: Rapport des tomographies de résistivité électriquehttp://hdl.handle.net/2268/172634
Title: Prospection géophysique de la zone faillée de Hockai dans la région de Malmedy: Rapport des tomographies de résistivité électrique
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<br/>Author, co-author: Hermans, Thomas; Nguyen, Frédéric
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<br/>Abstract: Ce rapport consiste en la présentation des résultats des prospections géophysiques par tomographie de résistivité électrique (ERT) menées sur la zone faillée de Hockai dans la région de Malmedy. Le but principal de ces investigations est de juger de la fracturation de la roche dans et en dehors de la Zone de Faille de Hockai (ZFH) et de mettre en évidence les structures liées à cette zone de failles.La ruée vers l'or technologiquehttp://hdl.handle.net/2268/172467
Title: La ruée vers l'or technologique
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<br/>Author, co-author: Lambert, Fanny